Reversible Phase Transformation Research Articles

ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH2 and synergetic catalysis mechanism

Transition-metal nanoparticles (NPs) can catalytically improve the hydrogen desorption/absorption kinetics of MgH2, yet this catalysis could be enhanced further by supporting NPs on carbon-based matrix materials. In this work, Co NPs with a uniform size of 10 nm loaded on carbon nanotubes (Co@CNTs) were synthesized in situ by carbonizing zeolitic imidazolate framework-67 (ZIF-67). The novel Co@CNTs nanocatalyst was subsequently doped into MgH2 to remarkably improve its hydrogen storage properties. The MgH2-Co@CNTs starts to obviously release hydrogen at 267.8 °C, displaying complete release of hydrogen at the capacity of 6.89 wt% at 300 °C within 15 min. For absorption, the MgH2-Co@CNTs uptakes 6.15 wt% H2 at 250 °C within 2 min. Moreover, both improved hydrogen capacity and enhanced reaction kinetics of MgH2-Co@CNTs can be well preserved during the 10 cycles, which confirms the excellent cycling hydrogen storage performances. Based on XRD, TEM and EDS results, the catalytic mechanism of MgH2-Co@CNTs can be ascribed to the synergetic effects of reversible phase transformation of Mg2Co to Mg2CoH5, and physical transformation of CNTs to carbon pieces. It is demonstrated that phase transformation of Mg2Co/Mg2CoH5 can act as “hydrogen gateway” to catalytically accelerate the de/rehydrogenation kinetics of MgH2. Meanwhile, the carbon pieces coated on the surfaces of MgH2 particles not only offer diffusion channels for hydrogen atoms but also prevent aggregation of MgH2 NPs, resulting in the fast reaction rate and excellent cycling hydrogen storage properties of MgH2-Co@CNTs system.

Lithium‐Ion Batteries: Atomic‐Scale Observation of Electrochemically Reversible Phase Transformations in SnSe2 Single Crystals (Adv. Mater. 51/2018)

In situ transmission electron microscopy is a powerful tool to capture spatiotemporal evolution of dynamic phenomena. In article number 1804925, Vinayak P. Dravid, Kai He, and co-workers implement this method to uncover atomic structural evolution of electrochemically reversible phase transformations during lithiation and delithiation of SnSe2, which is promising for lithium-ion batteries.

Revealing a conversion-alloying reaction mechanism behind high capacity and rate capability of SnS/N-doped graphene anode by in situ TEM

Developing an electrode material with improved ionic transport dynamics in a battery has been the focus of research. Here, we report a facile one-step hydrothermal synthesis method to prepare anode material of ultra-small SnS nanocrystals (NCs) anchored on N-doped graphene nanosheets (SnS/N-G), which is expected to significantly the dynamics of lithium transport, enabling an exceptional capacity of 1120.3 mAh g−1 at 0.1 A g−1 after 130 cycles and superior rate capabilities of 446.3 and 340.7 mAh g−1 at 2 and 3 A g−1, respectively. Furthermore, the lithiation/delithiation behaviors of SnS/N-G anode were observed in real time using in situ transmission electron microscopy to reveal the corresponding kinetics. By tracking the full lithiation procedure, in situ electron diffraction and high-resolution TEM imaging found that the original SnS phase was firstly transformed to Sn phase by conversion reaction and then to Li22Sn5 phase by alloying reaction. Notably, a stable and reversible phase transformation was established between Li22Sn5 and Sn phases during subsequent charge-discharge cycles. In the meantime, the volume expansion-induced pulverization of SnS NCs was evidently alleviated by graphene matrix that not only provided a two-dimensional support to buffer the volume change, but also improved the ion migration kinetics, as corroborated by superior rate capability.

An Ultrafast and Highly Stable Potassium-Organic Battery.

Potassium-organic batteries have a great potential for applications in the grid-scale energy storage owing to their low cost and abundant resources, although they suffer from the inferior cycle stability, fast capacity decay, and low power density. A highly reversible phase transformation of the organic cathode during potassiation/depotassiation is the key factor for the capacity retention, as revealed here. Consequently, the potassium-organic battery achieves a high power density of 9796 W kg-1 , a remarkable energy efficiency of 89%, a long cycle stability for 1000 cycles, a superior areal capacity around 2 mA h cm-2 , and a long-term cycling time over 8 months. Besides, the full cells also exhibit a superior rate performance and good cycle stability over 3000 cycles. This work provides new insight into the stabilization of the organic cathode, and demonstrates the enormous potential of organic cathodes for application in high-power potassium-ion batteries (PIBs), which may bring PIBs to new heights.

Atomic-Scale Observation of Electrochemically Reversible Phase Transformations in SnSe2 Single Crystals.

2D materials have shown great promise to advance next-generation lithium-ion battery technology. Specifically, tin-based chalcogenides have attracted widespread attention because lithium insertion can introduce phase transformations via three types of reactions-intercalation, conversion, and alloying-but the corresponding structural changes throughout these processes, and whether they are reversible, are not fully understood. Here, the first real-time and atomic-scale observation of reversible phase transformations is reported during the lithiation and delithiation of SnSe2 single crystals, using in situ high-resolution transmission electron microscopy complemented by first-principles calculations. Lithiation proceeds sequentially through intercalation, conversion, and alloying reactions (SnSe2 → Lix SnSe2 → Li2 Se + Sn → Li2 Se + Li17 Sn4 ) in a manner that maintains structural and crystallographic integrity, whereas delithiation forms numerous well-aligned SnSe2 nanodomains via a homogeneous deconversion process, but gradually loses the coherent orientation in subsequent cycling. Furthermore, alloying and dealloying reactions cause dramatic structural reorganization and thereby consequently reduce structural stability and electrochemical cyclability, which implies that deep discharge for Sn chalcogenide electrodes should be avoided. Overall, the findings elucidate atomistic lithiation and delithiation mechanisms in SnSe2 with potential implications for the broader class of 2D metal chalcogenides.

Concentrated electrolytes unlock the full energy potential of potassium-sulfur battery chemistry

Rechargeable K-S batteries provide a theoretical energy density almost double that of the current Li-ion batteries. However, in practice, this energy potential is largely compromised predominantly because of the dissolution and shuttle reactions of the redox intermediates in the commonly used electrolytes. Herein, we demonstrate for the first time that a 5 M concentrated electrolyte can effectively mitigate this parasitic effect and enables a full energy utilization of the K-S battery chemistry. The prototype K-S batteries operating at an average discharging voltage of 2.1 V deliver a discharging energy density of ~1270 Wh/kgS or ~700 Wh/kg(K2S3), which reaches the theoretical limit and is ~40%–100% higher than those of the current Li-ion batteries. X-ray powder diffraction analysis provides the first unambiguous evidence that the K-S battery chemistry involves reversible stepwise phase transformations of S8⇔K2S6⇔K2S5⇔K2S4⇔K2S3. This work shines a light on the full utilization and in-depth mechanistic understanding of high-energy-density K-S batteries.

Microstructural Characterization and Mechanical Behavior of NiTi Shape Memory Alloys Ultrasonic Joints Using Cu Interlayer.

NiTi shape memory alloys (SMAs) are a class of functional materials which can be significantly deformed and recover their original shape via a reversible martensitic phase transformation. Developing effective joining techniques can expand the application of SMAs in the medical and engineering fields. In this study, ultrasonic spot welding (USW), a solid-state joining technique, was used to join NiTi sheets using a Cu interlayer in between the two joining sheets. The influence of USW process on the microstructural characteristics and mechanical behavior of the NiTi joints was investigated. Compared with conventional fusion welding techniques, no intermetallic compounds formed in the joints, which is extreme importance for this particular class of alloys. The joining mechanisms involve a combination of shear plastic deformation, mechanical interlocking and formation of micro-welds. A better bonding interface was obtained with higher welding energy levels, which contributed to a higher tensile load. An interfacial fracture mode occurred and the fracture surfaces exhibited both brittle and ductile-like characteristics with the existence of tear ridges and dimples. The fracture initiated at the weak region of the joint border and then propagated through it, leading to tearing of Cu foil at the fracture interface.

Reversible Phase Transformation in Polycrystalline TRIP Steels Induced by Cyclic Indentation Performed at the Nanometric Length Scale

Metastable austenitic stainless steels are an interesting group of materials, which exhibit the Transformation Induced Plasticity effect. In this regard, phase transformation from austenite to martensite enhances the work hardening of the metastable austenitic stainless steels affecting the deformation dynamics and mechanical properties including fatigue properties. Within this context, the reversible load‐induced phase transformation from γ to ϵ‐martensite is investigated at the local scale under cyclic indentation. This reversible phase transformation is manifested itself by a combination of hysteresis loops, elbow formation, and reversible pop‐ins in the loading curve. The initial cyclic achieved through the nanoindentation technique allows to identify three different deformation regimes for the <111> austenitic grains. Firstly, a softening effect takes place due to the dislocation activation; subsequently the phase transformation induces a hardening effect and finally, the load deformation curve reaches a plateau where no more plastic deformation is observed.

Multiscale Deformation of Commercial Titanium and Ti一6Al-4V Alloy Subjected to Electron Beam Surface Treatment

The paper studies the effect of electron beam irradiation on the surface morphology, microstructure, and phase transformations in the surface layers of commercial titanium and titanium alloy Ti-6Nl-4V. It has been found that the plastic deformation of the modified surface layers under uniaxial tension of the studied specimens occurs at several scale levels. The main plastic deformation mechanism of the modified surface layers is shear induced by the maximum tangential stresses. The dislocation deformation mechanisms effectively accommodate the material rotation caused by the displacement of one part of the crystal relative to the other. Noncrystallographic shear banding in the surface layer with a nonequilibrium martensitic structure is associated with plastic lattice distortion and with reversible structural phase transformations.

Phase transformation induced reversible modulation of upconversion luminescence of WO3:Yb3+, Er3+ phosphor for switching devices.

The upconverting luminescence properties of phosphors are dependent on the hosts. In this work, the WO3:Yb3+, Er3+ phosphor was prepared, and the reversible phase transformation from the WO3 to the WO2 was obtained by alternating the sintering in a reducing atmosphere or in air. The influence of reversible phase transformation on the upconversion luminescence was investigated first. The WO3:Yb3+, Er3+ phosphor exhibits the visible upconversion luminescence, while no upconversion luminescence was observed in the WO2:Yb3+, Er3+ phosphor. The reversible modulation of upconversion luminescence of the WO3:Yb3+, Er3+ phosphor retains the excellent reproducibility, exhibiting the potential applications in data storage and optical switches.

Analytical model for the torsional response of superelastic shape memory alloy circular sections subjected to a loading-unloading cycle

This paper presents a new analytical model for superelastic shape memory alloy (SMA) circular shafts subjected to pure torsion. The model considers reversible phase transformation between austenite and a single martensite variant, driven by mechanical loading. The applied torque is gradually increased from zero to a value sufficient to cause complete transformation of the austenitic phase into martensite away from the center of the shaft. The torque is then gradually removed, resulting in complete strain recovery. At each stage of the loading/unloading process, an analytical relation is established between the torque and the angle of twist. Special emphasis is placed on modeling the unloading process, for which the proposed solution significantly improves upon existing work. The analytical model is validated against high-accuracy numerical solutions and finite element analysis results.

Destabilizing the dehydriding thermodynamics of MgH2 by reversible intermetallics formation in Mg−Ag−Zn ternary alloys

In this study, Mg−Ag−Zn ternary alloys are employed as carriers for hydrogen storage, which show improved thermodynamic and kinetic properties compared with pure Mg. Mg90Ag7.5Zn2.5 alloy exhibits a reversible phase transformation during hydriding-dehydriding process with a maximum hydrogen storage capacity of approximately 4.2 wt%. Two-step reactions occur in the dehydriding process of Mg90Ag7.5Zn2.5. In the first step, a portion of MgH2 reacts with MgAg and MgZn1.8Ag0.2 phases to transform to Mg54(Ag, Zn)17 solid solution and release hydrogen. After that, the remaining MgH2 decomposes to Mg. Because of the modified reaction pathway in the first step dehydrogenation, the dehydriding equilibrium pressure at 300 °C increases to 0.28 MPa. Besides, the apparent activation energy (Ea) for dehydrogenation of the Mg90Ag7.5Zn2.5 alloy is lowered to 118.7 kJ mol−1. By fully utilizing the first step reaction, another alloy Mg78Ag16.5Zn5.5 is designed, which presents intact destabilized dehydriding thermodynamics and a lower dehydrogenation Ea of 116.5 kJ mol−1. Our work demonstrates that alloying Mg with elements that can form compounds with low formation enthalpies, and the hydrides of the elements have low stabilities, are the main guidelines for the designing of the Mg-based hydrogen storage alloys with destabilized dehydriding thermodynamics by reversible intermetallics formation.

The advance of nickel-cobalt-sulfide as ultra-fast/high sodium storage materials: The influences of morphology structure, phase evolution and interface property

Numerous interests have been captured for bimetallic NiCo2S4 ascribed to its excellent electrical conductivity, whilst its sluggish sodium-ion kinetics at high-rate limits the advancement of reversible sodium storage. Herein, NiCo2S4 nanodots (~ 9 nm) uniformly incorporated with N-doped carbon are prepared (NiCo2S4@NC) through bottom-up strategy from 0D to stable structure. Considering that the suitable ether-based electrolyte (NaCF3SO3/DEGDME) may well promote faster sodium-ion transportation due to flexible one-dimensional chain structure and favorable solvent-salt interaction, and the optimal voltage region (0.4–3.0 V) could effectively successfully sidestep the side reaction and maintain reversible phase transformation. Such multi-factors tuned NiCo2S4@NC offers remarkable electrochemical performance as anode for SIBs. It delivers a stable capacity of 570.1 mAh g−1 after 200 cycles at 0.2 A g−1, and still retains 395.6 mAh g−1 at 6.0 A g−1 after 5,000 loops. Significantly, the mechanism and dynamics explorations by cyclic voltammetry (CV) profoundly reveal the dominant surface-capacitive behaviors of NiCo2S4@NC. A suite of in-situ electrochemical impedance spectroscopy (EIS) analyses further explore the regular dual-interface resistances of NiCo2S4@NC during the sodiation/desodiation process, corresponding to the reversible phase evolution and stable carbon matrix. This systematic study establishes a firm foundation for the later research of TMDs as excellent energy-storage anode materials for SIBs.

Zeolitic imidazolate framework-7: Novel ammonia atmosphere-assisted synthesis, thermal and chemical durability, phase reversibility and potential as highly efficient nanophotocatalyst

This is the first representation of novel sodalite zeolitic imidazolate framework-7 (ZIF-7) which has been made in ethanolic solution at room temperature via an ammonia atmosphere. High thermal stability up to 400 °C is representative of great persistence which has been proved by XRD and TG data. Chemical durability of the as-made ZIF-7 especially at boiled DMF exhibited by XRD patterns can present it as an interesting material without structural alteration after treatment in such harsh condition. Reversible phase transformation of ZIF-7 was totally checked by immersing in ethanol and DMF indicated that the framework can maintain its structural flexibility under heat and solvent treatment. Moreover, the “gate-opening” phenomenon performed by CO2 adsorption–desorption reveals structural breathing effect of ZIF-7 framework that makes it as potential material in CO2 adsorption/separation. In the end, the sacrificial metal-doped (Mn2+, Ni2+, Cu2+, Cd2+ and Ag+) ZIF-7 precursors were applied for preparation of their corresponded metal-doped ZnO as the heterogeneous catalyst to degrade Rhodamine-B (RhB) dye in water under UV-irradiation (up to 99% within 90 min by 0.5% Ag-ZnO (S15)). The recyclability experiment after 5 runs for the optimized catalyst demonstrated that the metal-doped ZnO can be operated consecutively without remarkable decreasing in its activity. These observations exhibit the excellent and beneficial properties of metal-doped ZnO can be as heterogeneous photocatalyst for the removal of organic contaminants in water.

Micrometer-Sized Nanoporous Sb/C Anode with High Volumetric Capacity and Fast Charging Performance for Sodium-Ion Batteries

Designing metal/C nanocomposites has been a prevalent strategy to address the volume expansion issue of alloying metal Na-ion battery (NIB) anodes but typically suffers from poor volumetric capacity. Here, micrometer-sized nanoporous Sb/C anode with high volumetric capacity and outstanding electrochemical performance is successfully synthesized using facile synthesis of a new class of solid-state reduction chemistry. The resulting Sb/C composite, containing 10 wt % C, possesses the combination of unique structural characteristics, including (1) micrometer-sized secondary particle, enabling high particle density; (2) nanoscale Sb crystallites, permitting reversible phase transformation during cycling; and (3) uniformly distributed nanoporosity, providing accommodation for Sb expansion and facile Na-ion diffusion. The Sb/C composite anode, showing outstanding cycling stability, exhibits a gravimetric capacity of 436 mAh g–1-(Sb+C), a volumetric capacity of 427 mAh cm–3 and over 80% capacity retention at nea...

Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO4- x: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4 undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4- x, in space group R3̅ m with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4- x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4- x. Cooling δ-SrUO4- x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4- x with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4- x to enable the transformation to δ-SrUO4- x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4- x at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4- x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4- x structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations.

Yolk-Shell Structured Sb@C Anodes for High Energy Na-Ion Batteries

Na-ion batteries (NIBs), as the analogy to Li-ion batteries, are regarded as one of the most promising technologies to satisfy the request on both performance and cost for grid energy storage. To further boost the specific energy and power of the state-of-the-art NIB, an anode material with comparable capacity and long-term stability to graphite anodes in lithium-ion batteries (LIBs) is highly desired. Antimony (Sb) has a theoretical capacity of 660 mAh g−1 upon full sodiation and hence holds great promises as the good anode candidate for high energy Na-ion batteries. A low cost and scalable approach of annealing poly-dopamine (PDA) coated Sb2O3 was used to prepare the yolk-shell structured Sb@C nanocomposite for high performance Na-ion battery anodes. Controlled reduction reaction and selective removal of Sb2O3 by acid etching generate the promised void space to accommodate the volume expansion of Sb while carbonization of the PDA forms highly conductive carbon shells. The by-product from the etching, SbCl3, can be reused rendering the approach a scalable and economic one. The yolk-shell Sb@C anode exhibits excellent electrochemical performance in terms of specific capacity, rate capability and cycling stability. The reversible phase transformation of crystalline Sb during charge/discharge was observed by the in-situ XRD for the first time. The designed composite with Sb: C of 70: 30 delivers almost theoretical capacity of ~ 554 mAh g−1. The specific capacity is still ~ 315 mhA g−1 at the current density of 5 A g−1 (10 C rate). The O3-Na0.9[Cu0.22Fe0.30Mn0.48]O2 (cathode)-Sb@C-hard carbon (anode) full-cell shows high specific energy of ~ 130 Wh kg−1 (based on the total weight of cathode and anode) in the voltage range of 2.0–4.0 V in NaClO4 electrolyte with 10 wt% fluoroethylene carbonate (FEC) additive. It is ~ 1.5 times energy of the full-cells with similar design using hard carbon anodes (~ 84 Wh kg−1). Figure 1

Effect of stress redistribution during thermal actuation of shape memory alloys in notched cylindrical bars

Shape memory alloys present a unique ability to undergo a solid-to-solid, diffusionless, reversible phase transformation. The forward phase transformation is commonly associated with transforming from the austenitic phase to the martensitic phase, while the reverse transformation is defined by going from the martensitic phase to the austenitic phase. In thermal actuation loading paths, forward transformation is generally associated with cooling, while reverse transformation is commonly associated with heating. In this article, however, it is shown that reverse transformation may occur during cooling of notched cylindrical shape memory alloy bars. The reversal in phase transformation is associated with the redistribution of stress in the shape memory alloy due to the phase transformation.

Influences of W Content on the Phase Transformation Properties and the Associated Stress Change in Thin Film Substrate Combinations Studied by Fabrication and Characterization of Thin Film V1- xW xO2 Materials Libraries.

The mechanical stress change of VO2 film substrate combinations during their reversible phase transformation makes them promising for applications in micro/nanoactuators. V1- xW xO2 thin film libraries were fabricated by reactive combinatorial cosputtering to investigate the effects of the addition of W on mechanical and other transformation properties. High-throughput characterization methods were used to systematically determine the composition spread, crystalline structure, surface topography, as well as the temperature-dependent phase transformation properties, that is, the hysteresis curves of the resistance and stress change. The study indicates that as x in V1- xW xO2 increases from 0.007 to 0.044 the crystalline structure gradually shifts from the VO2 (M) phase to the VO2 (R) phase. The transformation temperature decreases by 15 K/at. % and the resistance change is reduced to 1 order of magnitude, accompanied by a wider transition range and a narrower hysteresis with a minimal value of 1.8 K. A V1- xW xO2 library deposited on a Si3N4/SiO2-coated Si cantilever array wafer was used to study simultaneously the temperature-dependent stress change σ( T) of films with different W content through the phase transformation. Compared with σ( T) of ∼700 MPa of a VO2 film, σ( T) in V1- xW xO2 films decreases to ∼250 MPa. Meanwhile, σ( T) becomes less abrupt and occurs over a wider temperature range with decreased transformation temperatures.

Dynamic mechanical behaviors and failure thresholds of ultra-high strength low-alloy steel under strain rate 0.001/s to 106/s

In this study, the dynamic mechanical behaviors of the typical ultra-high strength low-alloy martensite steel 35CrMnSiA under strain rate 0.001/s～106/s were studied through quasi-static compressive tests (0.001/s), SHPB tests (2000/s～5000/s) and planar plate impact tests together with DISAR tests (105/s～106/s). The XRD analysis and metallographic observation were conducted to investigate the microstructure evolutions and failure mechanism of 35CrMnSiA under different stress states. Under uniaxial stress state, the adiabatic shear failure occurs as long as the rise rate of plastic strain energy density reaches 10.58×106 J·m−3 µ s−1 and the plastic strain energy density is above 4.51×108 J·m−3. For 35CrMnSiA samples under uniaxial strain state, the critical pressure for reversible phase transformation (α→ε, BCC→HCP) falls in the range of 17.57–19.19 GPa. Characterized by prominent temperature rise and volume shrinkage, the α→ε phase transformation induced by continuous dynamic recrystallization contributed to increasing the strength but weakening the ductility of 35CrMnSiA. In addition, the Hugoniot coefficients for 35CrMnSiA under a wide range of pressures have been determined. Moreover, the failure thresholds for 35CrMnSiA were obtained by dynamic fracture experiments and high velocity impact experiments: 35CrMnSiA projectiles fractured over impact pressure of 2.59 GPa, and when the impact pressure exceeded 21.25 GPa above which 35CrMnSiA suffered phase transformation, the projectiles had severe mass abrasion.